It is well known to produce semiconductor single crystal material using the Czochralski technique by forming a melt of the crystal material and bringing a seed crystal into contact with the melt. The seed is then pulled slowly upwards, the molten material solidifying at the seed-melt interface, thus forming a single crystal billet as the seed is continued to be pulled slowly upwards. Alternatively, of course, the billet can be formed by maintaining the seed in a fixed position while slowly lowering the melt relative to the position of the seed.
While such a method has been found to be quite effective, the crystal produced suffers from non-uniform electrical resistivity along its length. This is primarily due to the fact that the doping agents (e.g., arsenic, antimony, gallium or indium) commonly added to the pure semiconductor material (e.g., silicon, germanium), are more soluble in the liquid semiconductor material than in the solidifying or solid crystal. Hence, in a growing crystal, the concentration of doping agent in the melt keeps increasing as the crystal is pulled from the melt. This steadily increasing concentration of doping agent remaining in the melt also results in an increase of dopant and hence a decrease in resistivity along the length of the grown crystal billet as well as eventual saturation of the dopant in the melt. This saturation then results in the precipitation of a separate phase from the melt, which in turn provides nucleation sites for polycrystalline growth, thereby interfering with the continued, desired, single crystal growth. The problem of saturation and polycrystalline formation is especially significant in the growth of a heavily doped crystal wherein the dopant has a high segregation behavior.
The aforementioned problems have been addressed by a technique known as the double crucible method for Czochralski crystal growth. Generally, this method of pulling crystals from a melt employs an inner crucible from which the crystal is pulled, which inner crucible is positioned within an outer crucible containing a reservoir of material supplied to the inner crucible through an orifice connecting the two crucibles. This double crucible technique is employed to control the dopant concentration of the melt by, for example, providing a melt of relatively high concentration of dopant in the inner crucible and one of lesser dopant concentration or dopant-free material in the outer crucible. As the crystal is being pulled from the inner crucible, the lower concentration material of the outer crucible passes through the orifice connecting the crucibles and enters the inner crucible thereby maintaining a uniform dopant concentration in the inner crucible. This method is specifically suitable for the growth of crystals wherein the dopant is significantly more soluble in the melt than in the solid grown crystal. An example of such a crystal is antimony doped silicon. In Czochralski growth of such a crystal, as the crystal is being pulled from the melt, only a small percentage of the dopant enters the growing crystal while the remaining melt normally would tend to become more and more concentrated with dopant. However, with the use of a reservoir in the outer crucible which consists of a melt containing a low dopant concentration or a dopant-free melt which enters the inner crucible through the connecting orifice and replaces the amount of melt removed from the crucible by the growing crystal, one can approximately maintain the initial concentration of dopant in the inner crucible melt.
Notwithstanding the advantages gained by the use of double crucible techniques as opposed to the original single crucible Czochralski growth technique, it has been found that in the growth of certain crystals, such as heavily antimony doped silicon, e.g., on the order of 4.times.10.sup.18 atoms/cc, that while a relatively uniform resistivity of the billet can be achieved, typically only about 50-60% of the initial total charge of melt material can be pulled into a single crystal before polycrystalline growth is observed. It is therefore desirable to provide a method for pulling crystals of this kind in which substantially more than 50-60% of the initial charge is useful, thereby allowing the growth of larger crystals and further increasing the efficiency and economics of the crystal growth process. I have discovered that an important parameter in dealing with the aforementioned problem of limited useful melt charge is that of the relative ratios of the volumes of the inner crucible to the outer crucible as compared to the ratios of the cross-sectional areas of these crucibles. In the prior art double crucible apparatus, the ratio of the primary areas of the inner and outer crucibles as compared to the ratio of the volumes of the respective crucibles are generally either identical or the area ratio is greater than the volume ratio. This can be seen, for example, with reference to U.S. Pat. No. 2,892,739.